A personal audio device, such as a wireless telephone, may include a noise canceling circuit to reduce background noise in audio signals. One example of such a noise cancellation circuit is an adaptive noise cancellation circuit that adaptively generates an anti-noise signal from a reference microphone signal and injects the anti-noise signal into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone may also be provided proximate the speaker to measure the ambient sounds and transducer output near the transducer, thus providing an indication of the effectiveness of the noise canceling. A processing circuit uses the reference and/or error microphone, optionally along with a microphone provided for capturing near-end speech, to determine whether the noise cancellation circuit is incorrectly adapting or may incorrectly adapt to the instant acoustic environment and/or whether the anti-noise signal may be incorrect and/or disruptive and then take action in the processing circuit to prevent or remedy such conditions.
Examples of such adaptive noise cancellation systems are disclosed in published U.S. Patent Application 2012/0140943, published on Jun. 7, 2012, and Published U.S. Patent Application 2012/0207317, published on Aug. 16, 2012, both of which are incorporated herein by reference. Both of these references are assigned to the same assignee as the present application, and one names at least one inventor in common and thus are not “Prior Art” to the present application. However, they are provided to facilitate the understating of noise cancellation circuits as applied in the field of use. These references are provided by way of background only to illustrate one problem solved by the present invention. They should not be taken as limiting the present invention to any one type of multi-microphone application or noise cancellation circuit.
Referring now to FIG. 1, a wireless telephone 10 is shown in proximity to a human ear 5. Wireless telephone 10 includes a transducer, such as speaker SPKR that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ringtones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from web-pages or other network communications received by wireless telephone 10 and audio indications such as battery low and other system event notifications. A near-speech microphone NS is provided to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s).
Wireless telephone 10 includes active noise canceling circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R is provided for measuring the ambient acoustic environment and is positioned away from the typical position of a user's mouth, so that the near-end speech is minimized in the signal produced by reference microphone R. Prior art noise cancellation circuits rely on the use of two microphones (E and R). The embodiment of FIG. 1 also provides a third microphone, near-speech microphone NS, in order to further improve the noise cancellation operation by monitoring the ambient disturbance to the noise cancellation system when wireless telephone 10 is in close proximity to ear 5. Exemplary circuit 14 within wireless telephone 10 includes an audio CODEC integrated circuit 20 that receives the signals from reference microphone R, near speech microphone NS and error microphone E and interfaces with other integrated circuits such as an RF integrated circuit 12 containing the wireless telephone transceiver.
In general, the noise cancellation techniques measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and by also measuring the same ambient acoustic events impinging on error microphone E, the noise cancellation processing circuits of illustrated wireless telephone 10 adapt an anti-noise signal generated from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E. Since acoustic path P(z) (also referred to as the passive forward path) extends from reference microphone R to error microphone E, the noise cancellation circuits are essentially estimating acoustic path P(z) combined with removing effects of an electro-acoustic path S(z) (also referred to as secondary path) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which is affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, when wireless telephone is not firmly pressed to ear 5.
The dual microphone (R and NS) system of FIG. 1 is widely used in mobile telephony for uplink noise suppression. In order to protect the noise cancellation system, oversight software requires audio signals from R and NS microphones in order to detect certain situations, such as close talk, wind noise, howling, and the like. Close talk, as the term is known, occurs when the near-end user is talking while holding the phone to his/her ear. Wind noise occurs when wind buffets the microphone, producing loud buffeting noises. Howling occurs when an anti-noise signal is picked up by microphone R, and it is played out speaker SPKR. The speaker output gets coupled back to the reference microphone R and sets up a positive feedback loop. Howling can occur, for example, if a user cups their hand from the speaker back to the reference microphone R, or if there is some internal leakage path. Scratching is a term used to describe physical contact with a microphone, which produces a loud scratching noise.
Gain mismatch between the two microphones can reduce robustness and increase failures in detecting situations, such as close talk, scratch, howling and the like. If the gain from the two microphones differs, then the signal levels from the microphones will be different from one another, even when transmitting the same sound levels. In actual practice, some gain mismatch between the microphones is inevitable, due to manufacturing tolerances, microphone mounting and placement and the like. The absolute difference of amplitude frequency response could vary in a range of 0 to 10 dB or more.
Factory calibration of the microphones is one solution but provides only a partial solution to the problem. Microphone gain calibration provides only an overall gain calibration instead of a frequency response calibration. Moreover, even if calibrated at the factory, microphone response may drift over time.
Thus, it remains a requirement in the art to provide a way for calibrating a dual-microphone system when in use in the field, which provides a frequency response calibration in real-time.